Microfluidic flow manipulation device

ABSTRACT

Disclosed is an apparatus and method for the mixing of two microfluidic channels wherein several wells are oriented diagonally across the width of a mixing channel. The device effectively mixes the confluent streams with electrokinetic flow, and to a lesser degree, with pressure driven flow. The device and method may be further adapted to split a pair of confluent streams into two or more streams of equal or non-equal concentrations of reactants. Further, under electrokinetic flow, the surfaces of said wells may be specially coated so that the differing electroosmotic mobility between the surfaces of the wells and the surfaces of the channel may increase the mixing efficiency. The device and method are applicable to the steady state mixing as well as the dynamic application of mixing a plug of reagent with a confluent stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to U.S. provisionalapplication No. 60/323,509 entitled “MICROCHANNEL DESIGNS FOR MIXING ANDSPLITTING MICROFLUIDIC STREAMS UNDER ELECTROKINETIC OR PRESSURE DRIVENFLOW” filed Sep. 19, 2001 by Timothy J Johnson, David J Ross, and LaurieE Locascio, the entire disclosure is specifically incorporated herein byreference for all that it discloses and teaches.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The present invention pertains generally to microfluidic flow devicesand specifically to mixers and splitters of microfluidic flow.

b. Description of the Background

The application of microfluidic analytical devices to chemical orbiological assays has developed rapidly over the last decade. Althoughmicrofluidic devices have been highly successful, several performancelimitations exist, notably reagent mixing.

Most mixing devices rely on diffusive mixing, wherein the naturallaminar flow effects and the reagent's inherent diffusion coefficientcause the reagents to mix. Therefore, the mixing chamber/channel isusually extended to lengths that will ensure a completely mixed outletstream. This approach may be acceptable for low flowrates, but highflowrates (>1 cm/s) or low analyte diffusion coefficients (>10⁻⁷ cm/s²)will require excessively long mixing channels. The difficulty in rapidlymixing reagents results from the fact that the system is restricted tothe laminar flow regime (Re<2000) and also because the feature sizes aretoo small (typically<100 μm) to incorporate conventional mixingmechanisms.

The lack of turbulence in microfluidic systems has led to device designsthat utilize multi-laminate, or flow splitting techniques to accomplishmixing in channels of shorter length. These designs split the incomingstreams into several narrower confluent streams to reduce the mixingequilibrium time. Once mixing is complete, the narrow channels are thenbrought back together into a larger main channel for further transport,processing, and/or detection. The effectiveness of the flow splittingconcept is based on the fact that the equilibrium time scalesquadratically with the width of the channel. For example, if the widthof the channel decreases by two, then the equilibrium time and thechannel length decreased by a factor of four, or 25% of the originallength. However, even a mixing length of 25% may still be unsuitable forsome applications.

Other techniques for mixing may rely on active mechanical mixing, suchas stirring paddles and the like. For very small fluidic passages, suchdevices are extremely fragile and difficult to manufacture.

It would therefore be advantageous to provide a device and method ofmixing two confluent microfluidic laminar flows that did not require anexcessively long channel to effectively mix the flows. Further, it wouldbe advantageous to provide a splitting mechanism that may be able tosplit a stream of reagents into two streams of differing concentrations.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages and limitations of theprior art by providing a device and method for effectively mixing twoconfluent laminar reagents within a very short stream length. This isaccomplished by passing the confluent laminar flows over a series ofnarrow wells that are angled across the width of the channel. The deviceand method may also be used to split incoming streams into multiplestreams of equal or non-equal proportions. Additionally, the presentinvention may be used for the mixing of plugs of reagents whileminimizing axial dispersion of the reagent plug.

The present invention may therefore comprise a mixer of laminarmicrofluidic streams propelled by electrokinetic flow comprising: afirst inlet channel; a second inlet channel; a mixing channel startingat the confluence of the first inlet channel and the second inletchannel; and a plurality of wells disposed in the mixing channel, thewells being obliquely oriented substantially across the width of themixing channel.

The present invention may further comprise a splitter of a substantiallylaminar microfluidic stream comprising: a splitting channel coupled toat least two inlet ports and at least one outlet port in which thesubstantially laminar microfluidic stream has an axis of flow; and aplurality of wells disposed in the splitting channel, the wells beingoriented substantially longitudinally across the width of the channeland diagonally across the axis of flow, the wells being deeper inprofile than in width.

The present invention may further comprise a method of mixing twoconfluent laminar flows in microchannels comprising: providing a firstinlet stream and a second inlet stream that meet at a confluence pointto produce a confluent stream; passing the confluent stream through amixing channel, the mixing channel comprising a plurality of wells, thewells being oriented substantially longitudinally across the width ofthe mixing channel and diagonally across the mixing channel, the wellsbeing deeper in profile than in width; and producing a mixed laminarflow at the output of the mixing channel.

The advantages of the present invention are that flows may be combinedand mixed without the conventional long lengths of diffusive mixing. Thedevice may be further adapted to create two or more streams of equal ornon-equal proportions of reagents. The device may be adjusted to tunethe mix of the flows by adding various wells at different orientations,depths, and with various electroosmotic mobility coatings, all of whichmay have a substantial effect on the performance of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is an illustration of an embodiment of the present invention of amicrofluidic mixer.

FIG. 2A is a white light microscopy image of the embodiment of FIG. 1.

FIG. 2B is an image of the fluorescence of Rhodamine B introduced in thefirst inlet mixed with the buffer solution introduced in the secondinlet.

FIG. 2C is a similar image as FIG. 2B, except the flow is 0.81 cm/s.

FIG. 3 illustrates experimental results of the degree of mixing of theembodiment of FIG. 2A, wherein an electroosmotic flow of 0.06 cm/s wasachieved.

FIG. 4 illustrates the same experimental set up as FIG. 2C, with theelectroosmotic flowrate of 0.81 cm/s.

FIG. 5 illustrates the same experimental set up of FIGS. 3 and 4, with apressure driven flow.

FIG. 6 illustrates a second preferred embodiment of the presentinvention.

FIG. 7 is a white light microscopy image of the embodiment of FIG. 6.

FIG. 8 illustrates experimental results of the degree of mixing of tworeagents using embodiment of FIG. 7 and an embodiment similar toembodiment of FIG. 7 but with three wells instead of four.

FIG. 9 illustrates the results of the same experimental set up as FIG. 8with a higher electroosmotic flowrate of 0.81 cm/s.

FIG. 10 illustrates an embodiment of a stream splitter wherein two inletports form a confluent stream wherein the fluid on one half of thechannel is split into two streams located on opposite sides of thechannel that then exit through two outlets.

FIG. 11 illustrates an embodiment of a four-well mixer that was analyzedwith computational fluid dynamics techniques for variations in thepresent invention.

FIG. 12 illustrates some computational analysis of the flow patterns forvarious depths of wells, based on the embodiment shown in FIG. 11.

FIG. 13 illustrates some computational results of various angles of thewalls, based on the embodiment of FIG. 11.

FIG. 14A illustrates a plan view of the flow pattern of an embodiment ofthe present invention of a mixer with quantity 4 wells oriented at 15degrees off of the axis of flow.

FIG. 14B illustrates a cross sectional view of the flow pattern of FIG.14A, as observed from the cross section E-E.

FIG. 15 illustrates some computational results of changes in theelectroosmotic (EO) mobility of the surfaces of the wells.

FIG. 16A illustrates a plug of fluid introduced to and transported by achannel.

FIG. 16B illustrates an embodiment of the present invention wherein aplug of fluid is introduced into a channel in which four wells aredisposed.

FIG. 17 illustrates some results of a computational analysis of the flowof the embodiments of FIGS. 16A and 16B.

FIG. 18 illustrates the laser apparatus used to manufacture theexperimental devices referenced in the specification.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an embodiment 100 of the present invention of amicrofluidic mixer. Two inlet streams 102 and 104 are combined and mixedin the mixing region 106 to produce a mixed flow that exhausts out ofthe outlet 108. The mixing region 106 comprises several wells 110, 112,114, 116, 118, 120, 122, 124, 126, and 128 that are recessed into theoutlet 108.

In a first embodiment, the channels have a uniform, trapezoidal crosssection, with the width at the top being 72 μm, depth 31 μm, and thewidth at the bottom being 28 μm. The wells 110, 112, 114, 116, 118, 120,122, 124, 126, and 128 have a depth of 85 μm in the center of the well.

The wells 110, 112, 114, and 116 are parallel to each other and uniformin size. The wells are angled approximately 45 degrees from the axis ofthe outlet 108. The wells 118, 120, 122, 124, 126, and 128 areperpendicular to each other and are approximately 45 degrees from theaxis of the outlet 108.

The flow of reagents through the embodiment 100 may be electrokinetic,electroosmotic, or pressure driven flow. Since the electroosmotic flowis a wall driven phenomenon based on the surface charge of themicrochannel wall and on the local electric field, the fluid may enterand follow the contour of the wells, with the slanted well design usedto induce lateral transport across the channel. In a pressure drivenflow, the flow is not a wall driven phenomenon and therefore the fluidis not forced to enter the wells like electroosmotic flow. However, thepresence of the wells induces some lateral transport across the channelin pressure driven flow, although not as effective as withelectroosmotic flow. The presence of the wells under electrokineticflow, being the combination of both electroosmotic flow andelectrophoretic flow, will also enhance the mixing.

In an experiment with the present embodiment, a confluence of RhodamineB in a carbonate buffer was introduced into inlet 102 and carbonatebuffer was introduced into inlet 104. The fluorescence of the RhodamineB was measured to indicate the degree of mixing achieved by theembodiment 100.

The method of manufacture of the embodiment used in the experiment, aswell as the experimental setup and method of measurements, are givenelsewhere in this specification.

For the experiments, the length of each channel arm was 0.8 cm long, andthe dimensions of the channel are 72 μm wide at the top, 28 μm wide atthe bottom, and 31 μm deep. The laser etched wells that spanned acrossthe entire width of the channel had a depth of 85 μm in the center ofthe well relative to the bottom of the imprinted channel. The intensitymeasurement was taken at distance 130 or 443 μm from the beginning ofthe confluent region.

FIGS. 2A-C illustrate the results of the experiments as detailedelsewhere in this specification. FIG. 2A is a white light microscopyimage of the embodiment of FIG. 1. FIG. 2B is an image of thefluorescence of Rhodamine B introduced in the first inlet 202 mixed withthe buffer solution introduced in the second inlet 204, producing amixed stream 206. The flowrate is 0.06 cm/s. FIG. 2C is a similar imageas FIG. 2B, except the flow is 0.81 cm/s.

FIG. 3 illustrates the degree of mixing of the embodiment 100, whereinan electroosmotic flow of 0.06 cm/s was achieved. The horizontal axis isthe position across the width of the outlet 108 and the vertical axis isthe normalized intensity of the fluorescence of the Rhodamine B. Thecurve 302 represents the results of the measurement taken with no mixingwells present. The curve 304 represents perfect mixing. Curve 304 istrapezoidal in shape, following the profile of the trapezoidal outlet108. The curve 306 represents the actual experimental results. Thedetails concerning the experimental procedure and equipment used toperform all of the experiments referenced in this specification aregiven elsewhere in this specification.

FIG. 4 is a graph that illustrates the same experimental set up as FIG.3, with the electroosmotic flowrate of 0.81 cm/s. The line 402represents the results of the measurement taken with no mixing wellspresent. The line 404 represents perfect mixing. The line 406 representsthe actual experimental results.

From the results illustrated in FIGS. 2B, 2C, 3, and 4, the degree ofmixing exiting the mixer was 87.2% and 80.5% respectively, for theflowrates of 0.06 cm/s and 0.81 cm/s. To achieve the same degree ofmixing, theoretical predictions state that a channel length of 0.2 cmand 2.3 cm for electroosmotic flowrates would be required if no mixerwere present and based on diffusional mixing, assuming that thediffusion coefficient of the fluorescent material, Rhodamine B, is2.8×10⁻⁶ cm²/s. These results indicate that the length of the presentembodiment is 22% and 2% of the length of a comparable diffusive mixerfor the present application.

FIG. 5 is a graph that illustrates the results of the same experimentalset up of FIGS. 2, 3, and 4, with a pressure driven flow. Themeasurements were taken at 443 μm from the beginning of the confluenceregion. The curve 502 represents the perfect mixing results. The curve504 represents the experimental results with a pressure driven flow at0.21 cm/s. The curve 506 represents the experimental results with apressure driven flow at 1.25 cm/s. Since the pressure driven flow is nota wall driven phenomenon and therefore the fluid is not forced to enterthe wells like electroosmotic flow, the effects of the mixing are not asgreat with pressure driven flow as with electroosmotic flow. However,the presence of the wells does introduce some lateral transport acrossthe channel. This suggests that a series of wells could be optimized formixing under pressure driven flow.

FIG. 6 illustrates a second embodiment 600 of the present invention. Twoinlet streams 602 and 604 are combined and mixed in the mixing region606 to produce a mixed flow that exhausts out of the outlet 608. Themixing region 606 comprises several wells 610, 612, 614, and 616 thatare recessed into outlet 608. The measurement region 618 is 183 μm fromthe point of confluence of the inlets 602 and 604. The shape anddimensions of the channels and wells are the same as with embodiment 100of FIG. 1. The wells 610, 612, 614, and 616 are parallel to each other.

FIG. 7 is a white light microscopy image of an example of embodiment600.

FIG. 8 is a graph that illustrates the degree of mixing of two reagentsusing embodiment 600 and an embodiment similar to embodiment 600 butwith three wells instead of four. The experimental results of FIG. 8show results for electroosmotic flow of 0.06 cm/s taken 183 μm from thepoint of confluence of the inlet flows. The horizontal axis is theposition across the width of the outlet stream 608 and the vertical axisis the normalized intensity of Rhodamine B. The same experimental setupwas used for the results of FIG. 3 as FIG. 8, with the differences beingthe configuration of the mixing region and the position of themeasurements.

The curve 802 represents the mixing profile of the two inlet streamswhen no mixing wells are present. The curve 804 represents the perfectmixing of the two streams. The curve 806 represents the mixing profilefor the electroosmotic flowrate of 0.06 cm/s and a three well mixer. Thecurve 808 represents the mixing profile for the same flowrate and a fourwell mixer. The three well mixer is the embodiment 600 with well 616removed.

FIG. 9 illustrates the results of the same experimental set up as FIG. 8with a higher electroosmotic flowrate of 0.81 cm/s. The curve 902represents the mixing profile of the two inlet streams when no mixingwells are present. The curve 904 represents the perfect mixing of thetwo streams. The curve 906 represents the mixing profile for a threewell mixer. The curve 908 represents the mixing profile for a four wellmixer. The three well mixer is the embodiment 600 with well 616 removed.

The curve 908 forms two distinct humps, 910 and 912, indicating that thefour well mixer may be able to split the incoming streams into twostreams of similar concentrations. The number of wells, the shape,dimension, and placement of the wells may be adapted to providedifferent dilutions of the incoming fluid. Such adaptations may dependon the reagents and the diffusivity constants of the various componentsof the confluent streams. As such, the particular result desired, suchas splitting a stream or mixing a pair of confluent streams may beobtained by adjusting the quantity and position of the various wells.

FIG. 10 illustrates an embodiment 1000 of a stream splitter whereininlet port 1002 and inlet port 1003 form a confluent stream wherein thefluid that is on one half of the channel is split into two streams dueto the presence of the slanted wells, where the split streams arelocated on opposite sides of the channel that then exit through outlet1004 and outlet 1006. The flow of the microfluidic stream passes overthree wells 1008, 1010, and 1012 of similar design and construction asthose of other embodiments described in the present specification.

In lab-on-a-chip or μ-TAS (micro Total Analysis Systems), the use of aseries of wells within a microchannel may greatly enhance theeffectiveness of the entire system, especially when the system islimited to the laminar flow regime. The present invention is effectivefor low flowrates (<1 cm/s) as well as high flowrates (>1 cm/s). Thepresent invention is further able to effectively mix flows that aredriven electrokinetically, electroosmotically, or by pressure.

The present invention may be used to divide or split a stream intonon-equal or equal analyte concentrations. Such an application may beuseful in lab-on-a-chip or μ-TAS systems wherein streams of variousconcentrations are desired. Several embodiments of the present inventionmay be used in series, such that one stream is separated and split, thenseparated and split again, with the end result being several outletstreams with differing dilutions of the original incoming stream. Such asystem is known as serial dilution.

FIG. 11 illustrates an embodiment 1100 of a four-well mixer that wasanalyzed with computational fluid dynamics techniques for variations inthe present invention. The computational analyses were designed tocorrespond to the experimental results shown in the previous figures.The channel geometry and fluid properties were selected to closely matchthose of the experiments. The inlet 1102 contains a buffer fluid withRhodamine B that is mixed with a second inlet 1104 that contains onlythe buffer fluid. The fluid exits the embodiment 1100 via the outlet1105. The four wells 1106, 1108, 1110, and 1112 are located at an angletheta 1113 from the centerline axis of the mixer. The geometry of theembodiment 1100 is similar to the previous embodiments described herein.Cross-section line A 1114 will be used to illustrate the incomingstreams prior to mixing. Cross-section line B 1116 will be used toillustrate the mixing of the streams while in the well 1112, the last ofthe four wells. Cross-section line C 1118 will be used to illustrate themixing of the streams 5 μm past the exit of the last well. Cross-sectionline D 1120 will be used to illustrate the mixing of the streams at alocation of 420 μm past the point of confluence.

FIG. 12 illustrates some computational analyses of the flow patterns forvarious depths of wells, based on the embodiment 1100 shown in FIG. 11,with a constant well angle of 45°. The results for cross section A 1202illustrate the two incoming flows 1204 and 1206 prior to mixing. Theresults for cross section B 1208 illustrate the flow patterns for theflow within the last of the four wells. The 10 μm depth results 1210show that very little of the mixing is occurring in the well. The 50 μmdepth results 1212 show that a substantial portion of the mixing isoccurring in the well. The 85 μm depth results 1214 show that asubstantial portion of the mixing is occurring in the well, but thatthere is not much increase in the mixing due to the larger depth overthe 50 μm results 1212. These results indicate that there is a finitedepth wherein increasing the depth does not increase the degree ofmixing substantially. Further, these results illustrate that the wellsgreatly affect the mixing by forcing the fluids to fold over each other.

FIG. 13 illustrates the results of various angles of the wells asrepresented by the angle theta 1113 of FIG. 11. For all well anglesillustrated, the depth of the wells was held constant at 50 μm below thebottom of the imprinted channel. The results of cross section B-B 1302,cross section C-C 1304, and cross section D-D 1306 are shown in columns.The results for the various angled wells are shown in rows. Resultsalong row 1308 are for wells at a right angle or 90 degrees to the axisof flow. Results along rows 1310, 1312, and 1314 are for wells at 60degrees, 30 degrees, and 15 degrees to the axis of flow. The resultsindicate that a decreased angle of the well achieves a higher degree ofmixing.

The results along row 1308 for right angle wells show that there is nolateral transport across the width of the well. As the angle of thewells is decreased, there is increased lateral transport to the pointwhere the flow may be folded over on top of itself more than once. Thefolding action is an important mechanism that causes efficient mixing.

FIG. 14A illustrates a plan view of the flow pattern of an embodiment1400 of a mixer with quantity 4 wells oriented at 15 degrees off of theaxis of flow, and well depths set to 50 μm below the bottom of theimprinted channel. FIG. 14B illustrates a cross sectional view of theflow pattern of FIG. 14A, as observed from the cross section E-E. Theflow lines 1402 and 1404 illustrate that the fluid may exit the firstwell 1406 and reenter another well 1408 and thereby may fold during thepassage through the mixer 1400.

FIG. 15 illustrates the results of changes in the electroosmotic (EO)mobility of the surfaces of the wells. Different manufacturing processesmay create different EO mobilities on various surfaces of the channels.For example, of the manufacturing processes described for theexperiments described elsewhere in this specification, imprinting achannel has been shown to yield a different EO mobility than the laserablation manufacturing method. Further, other methods such aspolyelectrolyte multilayers, surface chemistry modifications, EOmobility suppression coatings, and other methods may be usedindividually or in combination to selectively change the EO mobility ofselective surfaces of the mixer.

The results of FIG. 15 illustrate the effects of increasing the EOmobility of the surfaces of the wells with respect to the EO mobility ofthe remaining surfaces of the mixer. The results are for a four wellmixer with 45 degree wells at a depth of 50 μm below the bottom of theimprinted channel. The column 1502 illustrates the results for sectionB-B, column 1504 illustrates the results for section C-C, and column1506 illustrates the results for section D-D, all of which relate thecross sections illustrated in FIG. 11.

For the purposes of this discussion, a ratio of the EO mobility of thewells divided by the EO mobility of the remainder of the surfaces willbe r_(EOM). The row 1508 illustrates the results when r_(EOM) is 1.24.Row 1510 illustrates the results for r_(EOM) of 2.00 and row 1512illustrates the results for r_(EOM) of 3.00. Row 1508 is illustrative ofthe approximate r_(EOM) of the experimental results described in FIGS.7, 8, and 9. The results indicate that as the r_(EOM) is increased,mixing can be enhanced. In other words, the increase of the EO mobility,by different manufacturing processes, selectively applied coatings, orother methods may dramatically increase the performance of a mixer ofthe present invention.

A use for the present invention is the mixing of plugs of fluid.Applications for such a use may be for lab on chip applications whereinseveral samples of fluid may be analyzed in succession. It would bedesirable for the plugs of fluid to be efficiently mixed, but tominimize the axial dispersion of the plug.

FIG. 16A illustrates a plug of fluid 1602 introduced into the channel1604. The channel 1604 illustrates a case wherein the wells of thepresent invention are not present and represents a baseline case. Themixed plug 1606 is shown downstream.

FIG. 16B illustrates an embodiment of the present invention wherein aplug of fluid 1608 is introduced into a channel 1610 in which four wells1612, 1614, 1616, and 1618 are disposed. The mixed plug 1620 is showndownstream. For FIG. 16B, r_(EOM) is set to 2.00.

FIG. 17 illustrates the results of a computational analysis of the flowof the embodiments of FIGS. 16A and 16B. The curves 1702 and 1704illustrate the average concentration of the plug as it passes the outletof the mixing channel over time. Curve 1702 represents the plug of fluidfrom FIG. 16A and curve 1704 represents the plug of fluid from FIG. 16B,with r_(EOM) equal to 2.00.

The cross section 1706 represents the analysis results for the point1708 and cross section 1710 represents the analysis for the point 1712.Both cross sections are the approximate high point of the concentration.For cross section 1706, the plug flow with no wells, the averageconcentration of the reagent is approximately 28% higher than the crosssection 1710. However, the standard deviation, an approximate measure ofthe degree of mixing, is approximately 3.6 times higher for crosssection 1706 wherein no wells were present. The lower standard deviationof the mixture that passed through the inventive wells indicates thatthe plug of reagent was very well mixed. Further, from the curve 1712,the plug of fluid is still intact, although slightly elongated whencompared to the reagent that was not passed over the inventive wells.

The present invention is a passive device that greatly enhances themixing of reagents under electrokinetic flow, and to a lesser degree,under pressure driven flow. The present invention significantlydecreases the channel length required for mixing reagents by placingwells in the flow channel at oblique angles to the axis of flow. Thewells may be of various depths, however, for a given set of reagents,flowrates, and channel geometries, there may be an optimum depth of awell wherein an increased depth may not increase the mixingeffectiveness.

Electroosmotic flow is a surface driven mechanism that may be enhancedto change the performance of the present invention. For example,increasing the electroosmotic mobility of selective surfaces such as thewells has been shown to increase the effectiveness of the mixer.

The manufacturing process and equipment used in the experimentsreferenced in this specification are herein defined.

Reagents and Materials. Laser Grade Rhodamine B was used as supplied byAcros Organics (Belgium) and dissolved in 20 mM, pH 9.4 carbonate bufferto a final concentration of 0.11 mM Rhodamine B. The buffer solution wasmade using deionized water from a Millipore Milli-Q system (Bedford,Mass.), and was filtered before use with a syringe filter (pore size0.22 μm).

Microchannels were made using polycarbonate sheet (PC; Lexan, GE Co.,Mt. Vernon, Ind.). Poly(ethylene terephthalate glycol) (PETG; Vivak, DMSEngineering Plastic Products, Sheffield, Mass.) was used to cover andseal the microchannel substrate. The glass transition temperature of PCand PETG are approximately 150° C. and 81° C., respectively.Polycarbonate was chosen as the substrate material because it has a highabsorption cross section to 248 nm light (the wavelength of the excimerlaser), therefore ablated structures have minimal surface roughness (<5nm). PETG was chosen to seal the microchannels because its glasstransition temperature is well below that of PC. Therefore, thermalsealing can be performed at a temperature that does not cause distortionof the PC microchannel.

Hot Imprinting Method. Prior to imprinting, the PC substrate was blownclean with ionized air. Channels were hot imprinted in the substratematerial using a silicon stamp with a trapezoidal-shaped raisedT-channel. The PC was place over the silicon stamp, the two items werethen placed between two aluminum heating blocks, and then thetemperature was raised to 155° C. Next, the assembly was placed in ahydraulic press and a pressure of 13.8 MPa (2000 psi) was applied for1.5 hours. The imprinted substrate was then removed from the templateand allowed to cool to room temperature. Channel dimensions weremeasured by optical profilometry.

Laser Ablation Method. A 248 nm excimer laser system (LMT-4000, PotomacPhotonics, Inc., Lanham, Mad.) was used to ablate microstructures withinthe pre-formed PC microchannel. The excimer laser system, FIG. 18,contains a laser light source 1, a round aperture (200 μm diameter) 2for delimiting the size and shape of the beam 3, a focusing lens (10×compound) 4, a visible light source 5, a CCD camera to image theablation process 6, and a controllable X-Y stage 7 with a vacuum chuck 8to hold the substrate 9 in place. Also, a nozzle 10 was present to sweepnitrogen over the substrate 9 during processing, and a vacuum nozzle 11was located on the opposite side of the stage to remove debris. For theexperiments conducted here, the size-delimiting aperture was chosen suchthat the ablated features would be smaller than the dimensions of thechannel. Also, the X-Y stage was moved linearly at a rate of 1 mm/s, andthe ablated wells were at a 45° angle relative to the axis of the mainchannel. The average power level per pulse was set to 2.04 μJ+/−0.14 μJ.The frequency of pulses was set to 200 Hz, with a constant pulse widthof 7 ns. The light after being focused exposed a circular area of1.90×10⁻⁶ cm².

Measuring Well Depth and Profile. The depth of the ablated wells wasmeasured by cutting the substrate with a microtome (Microm HM335 E,Walldorf, Germany) either perpendicular to the axis of the outletchannel or parallel to the slanted wells. The substrate was cut so thatthe edge of the substrate was within a few microns of the wells. Thewells were then imaged and measured using white light microscopy.

Microchannel Sealing Procedure. The pre-formed microchannels werecovered and thermally sealed with a flat piece of PETG (referred to asthe ‘lid’ throughout the rest of the text) of similar dimensions to thePC. Prior to bonding, the lid and the channel were cleaned withcompressed nitrogen gas. The lid was then placed on top of the channel,and the two pieces were clamped together between microscope glass slidesand bonded by heating in a circulating air over at 90.0° C.+/−0.5° C.for 13 minutes. It is important to keep the time and temperature as lowas possible in the sealing process to avoid physical alteration of themicrochannel.

For the electroosmotic flow studies, 3 mm diameter circular holes in thelid provided access to the channels and served as fluid reservoirs. Forthe pressure driven flow studies, 0.8 mm diameter circular holes in thelid, located at the ends of each inlet channel, provided access toinsert a section of hollow stainless-steel tubing. A 3 mm diameter holein the lid at the end of the outlet channel served as a waste reservoir.For each experiment, the channel arms were fixed to a length of 8 mm.

Flow Image Acquisition. Flourescence imaging of the rhodamine dye wasperformed using a research fluorescence microscope equipped with a 10×objective, a mercury arc lamp, a rhodamine filer set, and a video camera(COHU, San Diego, Calif.). Digital images were acquired using ScionImage™ software and a Scion LG-3 frame grabber (Scion, Inc., Frederick,Md.). For each experiment, images were captured every 1/60^(th) of asecond over a duration of 0.67 s, averaged, then recorded.

Experimental Set-up. To image the mixing under electroosmotic flow, themicrochannels were initially filled with the carbonate buffer solution.Then, an equal amount (typically 40 μL) of buffer was placed in oneinlet channel reservoir and in the outlet channel reservoir, while thesecond inlet reservoir was filled with the rhodamine-labeled buffer.Platinum electrodes were then placed in contact with the solution in thereservoirs such that the two inlet reservoirs were fixed to ground andthe potential was applied to the outlet channel reservoir. Themicrochannel was placed beneath the fluorescence microscope described inthe previous section, and images were acquired at several differentapplied voltages (0 to -1750V), beginning with zero applied voltage toverify that there was minimal flow resulting from hydrostatic pressure.The current through the microchannel was determined by measuring thevoltage drop across a 100 kΩ resistor (typically less than 1/1000 theresistance of the microchannel) connected to the high voltage supply inseries with the microchannel. For pressure driven flow studies, aprogrammable syringe pump (Harvard Apparatus PHD 2000, Holliston, Mass.)was interfaced to the stainless tubing in the inlet reservoirs viaTeflon tubing.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and various modifications as are suited to theparticular use contemplated. It is intended that the appended claims beconstrued to include other alternative embodiments of the inventionexcept insofar as limited by the prior art.

1. A mixer of laminar microfluidic streams propelled by electrokineticflow comprising: a first inlet channel; a second inlet channel; a mixingchannel starting at the confluence of said first inlet channel and saidsecond inlet channel; and a plurality of substantially straightunconnected wells disposed in said mixing channel, said wells beingobliquely oriented substantially across the width of said mixingchannel.
 2. The mixer of claim 1 wherein alternating wells areconfigured perpendicular to each other.
 3. The mixer of claim 1 whereinsaid wells are configured parallel to each other.
 4. The mixer of claim1 wherein at least a portion of the surfaces of said wells have anelectroosmotic mobility that is different from the electroosmoticmobility of at least a portion of said mixing channel.
 5. A splitter ofa substantially laminar microfluidic stream comprising: a splittingchannel coupled to at least two inlet ports and at least one outlet portin which said substantially laminar microfluidic stream has an axis offlow; and a plurality of substantially straight unconnected wellsdisposed in said splitting channel, said wells being orientedsubstantially longitudinally across the width of said channel anddiagonally across said axis of flow said wells being greater in depththan in width.
 6. The splitter of claim 5 wherein alternating wells areconfigured prependicular to each other.
 7. The splitter of claim 5wherein said wells are configured parallel to each other.
 8. Thesplitter of claim 5 wherein said microfluidic streams are propelled bypressure.
 9. The splitter of claim 5 wherein said microfluidic streamsare propelled by electroosmosis.
 10. The splitter of claim 5 whereinsaid microfluidic streams are propelled by electrokinetics.
 11. Thesplitter of claim 5 wherein at least a portion of the surfaces of saidwells have an electroosmotic mobility that is different from theelectroosmotic mobility of at least a portion of said splitting channel.12-18. (canceled)